Polishing system with in-line and in-situ metrology

ABSTRACT

A computer-implemented method for process control in chemical mechanical polishing in which an initial pre-polishing thickness of a substrate is measured at a metrology station, a parameter of an endpoint algorithm is determined from the initial thickness of the substrate, a substrates is polished at a polishing station, and polishing stops when an endpoint criterion is detected using the endpoint algorithm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional and claims the benefit of priorityunder 35 U.S.C. Section 120 of U.S. application Ser. No. 11/923,082,filed Oct. 24, 2007, which is a divisional of U.S. application Ser. No.11/467,116, filed Aug. 24, 2006, now U.S. Pat. No. 7,294,039, which is adivisional of U.S. application Ser. No. 11/166,022, filed Jun. 23, 2005,now U.S. Pat. No. 7,101,251, which is a continuation of U.S. applicationSer. No. 10/330,687, filed Dec. 27, 2002, now U.S. Pat. No. 6,939,198,which claims priority to U.S. Provisional Application Ser. No.60/344,411, filed on Dec. 28, 2001, the entire contents of which areincorporated herein by reference.

BACKGROUND

The present invention relates generally to chemical mechanical polishingof substrates, and more particularly to methods and apparatus fordetecting a polishing endpoint during a chemical mechanical polishingoperation.

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon wafer. Integrated circuits are typically formed onsubstrates, particularly silicon wafers, by the sequential deposition ofconductive, semiconductive or insulative layers. After each layer isdeposited, it is etched to create circuitry features. As a series oflayers are sequentially deposited and etched, the outer or uppermostsurface of the substrate, i.e., the exposed surface of the substrate,becomes increasingly nonplanar. This nonplanar surface presents problemsin the photolithographic steps of the integrated circuit fabricationprocess. Therefore, there is a need to periodically planarize thesubstrate surface. In addition, planarization is often needed to removea filler layer until an underlying stop layer is exposed, or to create alayer with a defined thickness.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. Conventionally, theexposed surface of the substrate is placed against a rotating polishingpad, although a linear belt or other polishing surface can be used. Thepolishing pad may be either a “standard” pad or a fixed-abrasive pad. Astandard pad has a durable roughened surface, whereas a fixed-abrasivepad has abrasive particles held in a containment media. The carrier headprovides a controllable load on the substrate to push it against thepolishing pad. A polishing slurry, including at least onechemically-reactive agent, and abrasive particles if a standard pad isused, is supplied to the surface of the polishing pad (also, somepolishing processes use a “nonabrasive” process).

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness or whether an underlying layer has beenexposed. If an excessive amount of material is removed (overpolishing),the substrate is rendered unusable. If, on the other hand, if aninsufficient amount of material is removed (underpolishing), thesubstrate must be reloaded into the CMP apparatus for furtherprocessing. This is a time-consuming procedure that reduces thethroughput of the CMP apparatus.

The polishing rate is sensitive to the slurry composition, the polishingpad condition, the relative speed between the polishing pad and thesubstrate, and the load on the substrate, as well as the initialsubstrate topography. In addition, there can be variations in thethickness of the layers in the substrate layers. These variations causevariations in the time needed to reach the polishing endpoint.Therefore, the polishing endpoint cannot be determined merely as afunction of polishing time.

Various methods are used to monitor and control the CMP planarity andlayer thickness during polishing. For example, the substrate thicknesscan be monitored in-situ by an optical sensor, such as aninterferometer. Alternatively, exposure of an underlying layer and theassociated change in reflectivity of the substrate can be detected areflectometer. In addition, various method are used to measure the layerthickness before or after thickness (e.g., in an in-line metrologystation). For example, a spectrometer, such as the NovaScan 210,manufactured by the Nova Corporation of Israel, can be used an in-linemetrology device.

Although these techniques are satisfactory, there is still room forimprovement in the accuracy of the determination of the polishingendpoint.

SUMMARY

In one aspect, the invention is directed to a computer-implementedmethod for process control in chemical mechanical polishing. The methodincludes measuring an initial pre-polishing thickness of a substrate ata metrology station, determining a parameter of an endpoint algorithmfrom the initial thickness of the substrate, polishing a plurality ofsubstrates at a polishing station, and stopping polishing when anendpoint criterion is detected using the endpoint algorithm.

Implementations of the invention may include one or more of thefollowing features. Each of the plurality of substrates may be monitoredin-situ during polishing and generating a monitoring signal. Theendpoint algorithm may include processing the monitoring signal todetect a signal feature indicating a final or intermediate endpoint. Theparameter may represent an initial delay time before the processing stepbegins, an overpolish time after detection of the signal feature, atotal polishing time, or a number of interference cycles. A polishingrate may be determined from the monitoring signal, and the parameter maybe adjusted based on the polishing rate. The adjusting step may occurafter a predetermined amount of an expected polish time has elapsed. Thefirst substrate may be one of the plurality of substrates.

In another aspect, the invention is directed to a computer-implementedmethod for process control in chemical mechanical polishing. The methodincludes measuring an initial pre-polishing thickness of a monitorsubstrate at a metrology station, determining a delay parameter of anendpoint algorithm from the initial thickness of the monitor substrate,polishing a product substrate at a polishing station, waiting for thedelay parameter once polishing of the product substrate has commenced,monitoring the product substrate in-situ during polishing after thedelay step to detect the endpoint criterion, and stopping polishing whenthe endpoint criterion is detected.

In another aspect, the invention is directed to a computer-implementedmethod for process control in chemical mechanical polishing. The methodincludes polishing a monitor substrate, monitoring the monitor substratein-situ during the polishing step to determine a lot representativeremoval rate, measuring an initial substrate thickness of a productsubstrate at a metrology station, calculating an endpoint time from theinitial substrate thickness and the lot representative removal rate,polishing the product substrate, and stopping polishing of the substrateat the endpoint time.

In another aspect, the invention is directed to a computer-implementedmethod for process control in chemical mechanical polishing. The methodincludes measuring an initial substrate thickness of a product substrateat a metrology station, polishing the product substrate, monitoring theproduct substrate in-situ during polishing to determine a polishingrate, calculating an endpoint time from the initial thickness and thepolishing rate, and stopping polishing of the substrate when theendpoint time is detected.

In another aspect, the invention is directed to a computer-implementedmethod for process control in chemical mechanical polishing. The methodincludes measuring an initial thickness of a product substrate at ametrology station, determining a target amount to remove from theinitial substrate thickness and a selected target thickness, determiningan endpoint criterion based on the amount to remove, monitoring thesubstrate with an in-situ monitoring system during polishing to detectan actual thickness removed from the substrate, and stopping polishingwhen the target amount to remove is detected.

In another aspect, the invention is directed to a computer-implementedmethod for process control in chemical mechanical polishing of asubstrate that includes an upper layer with an exposed surface coveringa lower layer. The method includes measuring a thickness of the lowerlayer of a substrate at a metrology station, determining an overpolishtime from the thickness of the under-layer wafer, polishing the exposedsurface of the substrate, monitoring the substrate with an in-situmonitoring system to detect an intermediate endpoint criterion, andstopping polishing of the substrate when the overpolish time has elapsedafter the intermediate endpoint criterion has been detected.

In another aspect, the invention is directed to a computer-implementedmethod for process control in chemical mechanical polishing. The methodincludes polishing a monitor substrate, monitoring the monitor substratewith an in-situ monitoring system to detect an intermediate endpointcriterion representing an underpolished state of the substrate, stoppingpolishing of the monitor substrate when the intermediate endpoint isdetected, measuring a post-polishing thickness of the monitor substrateat a metrology station, calculating an overpolish time from a differencebetween the post-polishing thickness and a desired thickness, polishinga product substrate, monitoring the product substrate with an in-situmonitoring system to detect the intermediate endpoint criterion, andstopping polishing of the substrate when the overpolish time has elapsedafter the intermediate endpoint criterion has been detected.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic plan view of a substrate processing system.

FIG. 2 is a schematic cross-sectional view of a polishing station in thesubstrate processing system.

FIG. 3 is a flowchart illustrating a method according to the invention.

FIG. 4 is a flowchart illustrating another method according to theinvention.

FIG. 5 is a flowchart illustrating another method according to theinvention.

FIG. 6 is a flowchart illustrating another method according to theinvention.

FIG. 7 is a flowchart illustrating another method according to theinvention.

FIG. 8 is a flowchart illustrating another method according to theinvention.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, a substrate processing system 20 includes achemical mechanical polisher 22, a wet robot 24, a cleaner 26, a factoryinterface module 28, and an in-line metrology station 30. Substrates 10are transported to the substrate processing system 20 in cassettes 12,and are extracted from the cassettes 12 by a robot 18 in the factoryinterface module 28 for transport to the polisher 22 or the cleaner 26.The operations of the substrate processing system 20 are coordinated bycontroller 32, which may include one or more programmable digitalcomputers executing distributed control software. A description of asimilar substrate processing system may be found in U.S. patentapplication Ser. No. 09/543,858, the entire disclosure of which isincorporated herein by reference.

The polisher 22 can be a Mirra® chemical mechanical polishermanufactured by Applied Materials, Inc. of Santa Clara, Calif. Adescription of a polisher may be found in U.S. Pat. No. 5,738,574, theentire disclosure of which is incorporated herein by reference. Anexemplary polisher includes three polishing stations 25 a, 25 b and 25c, and a transfer station 27. At each polishing station, a substrateundergoes a polishing process defined by multiple polishing parameters,such as the rotation rate of the polishing pad, the rotation rate of thecarrier head, the slurry composition and slurry flow rate, the pressureapplied to the substrate, and the like. These polishing parameters canbe controlled by the controller 32.

Referring to FIG. 2, a polishing station in the polisher includes arotatable platen 34 that supports a polishing pad 35, e.g., a standardor a fixed-abrasive or polishing pad. A support structure 38 supports acarrier heads 14 that holds a substrate 10 against the polishing pad 35.An aperture 37 is formed in the platen 34, and a transparent window 36is formed in a portion of the polishing pad 35 overlying the aperture.An optical monitoring system 40, which can function as a reflectometeror interferometer, is secured to the platen 27 beneath the aperture 37and rotates with the platen 34. The optical monitoring system 40includes a light source 44, such as a laser, and a detector 46, such asa photodiode. The light source generates a light beam 42 whichpropagates through transparent window 36 to impinge upon the exposedsurface of the substrate 10. The intensity of a reflected beam 48 fromthe substrate 10 is measured by the detector 46.

In operation, the polisher 22 uses the optical monitoring system 40 todetermine the amount of material removed from or remaining on thesurface of the substrate during polishing, to determine when the surfacehas become planarized, or to determine when an underlying layer has beenexposed. The light source 44 and the detector 46 are coupled to thecontroller 32. The controller 32 may be a general purpose digitalcomputer programmed to activate the light source 44 when the substrategenerally overlies the window, to store intensity measurements from thedetector 46, to display the intensity measurements on an output device49, to sort the intensity measurements into radial ranges, and to applythe endpoint detection logic to the measured signals to detect thepolishing endpoint.

Although the controller for the polishing system, the in-line metrologydevice and the in-situ endpoint detection system are shown as the samecomputer, these systems can be implemented as a distributed system, oras a set of independent systems that communicate using a preselectedprotocol. Moreover, various steps in the methods set forth below neednot be performed on the same controller system. For example, themeasurement of the initial layer thickness could be forwarded from thecontroller for the in-line metrology device to the controller for thepolishing system. The controller for the polishing system couldcalculate values for the variables used by the endpoint detectionalgorithm, and then forward the calculated values to the controller forthe in-situ endpoint detection system.

Although an optical monitoring system has been illustrated, the presentprocess can work with other in-situ monitoring systems, such as eddycurrent, capacitive or vibration sensing systems.

Returning to FIG. 1, the in-line metrology station 30 includes ametrology device 60. The in-line metrology station is located at aposition separate from the polishing stations 25 a-25 c, where thesubstrate can be placed before or after polishing. For example, thein-line metrology station 30 can be positioned off the factory interfacemodule 28 and accessible by the interface robot 18 as shown, or it canbe positioned between the polisher 22 and the factor interface moduleand be accessible to the wet robot 24, or it can be positioned in theinput stage 62 or the output stage of the cleaner 26. In general, themetrology device 60 is capable of measuring the thickness of one or morelayers in the substrate, including underlying layers. The metrologydevice can be a spectrometer, such as the NovaScan 210 manufactured byNova Corporation of Israel. Other suitable spectrometers that canmeasure the thickness of layers on the substrate 10 includespectrometers manufactured by KLA-Tencor Corporation and ThermawaveCorporation. Since the metrology device 60 operates in-line rather thanin-situ, it may be capable of higher accuracy measurements than theoptical monitoring system 40.

The overall approach of this invention to obtaining an accuratepolishing endpoint is to premeasure the initial thickness of thesubstrate, or a layer in the substrate, by the in-line metrology device60 prior to the substrate being polished, and use this data to adjustthe endpoint polishing routine.

FIG. 3 illustrates a method 100 according to one implementation of theinvention. First, the initial thickness of a layer on the substrate 10is measured by the in-line metrology device 30 (step 110). Themeasurement of the layer thickness can be aligned with the criticalareas on the substrate 10. The initial thickness is stored in a memoryof the controller 32. The controller 32 then calculates an initial deadtime (IDT) based on the initial layer thickness (step 120). The initialdead time can be calculated by dividing the difference between the anideal starting thickness for the endpoint algorithm and the measuredinitial thickness by an estimated polishing rate.

Next, the controller 32 modifies the endpoint algorithm using theinitial dead time. Specifically, the controller adjusts the algorithm sothat data collection using the in-situ endpoint detection system 40begins after expiration of the initial dead time from the start ofpolishing of the substrate. During polishing, the substrate 10 is placedin contact with the polishing pad 35 and monitored in-situ by themonitoring system 40 (step 140). As noted however, data collection fromthe monitoring system does not being until after the initial dead timehas expired. Once the endpoint detection system begins operating, thecontroller applies endpoint detection logic to the trace from themonitoring system to detect the endpoint of the polishing process (step150). Once the endpoint criteria are detected, polishing halts (step160).

The measurement of the substrate layer thickness and calculation of theIDT can be performed for just the first substrate in a cassette.Subsequently substrates from the same cassette can be polished using thesame IDT and adjusted endpoint algorithm.

A potential advantage of this system is that it can prevents theendpoint detection system from triggering too early or too late.Specifically, when a layer having a large thickness, such as oxide layerhaving a thickness of more than 4000 Angstroms, is to be removed, themethod prevents “order skipping”, i.e., mistakes by the endpointdetection system when the number of interference cycles is larger thanexpected. Since the endpoint detection algorithm does not begin until apoint near where the expected number of interference cycles remain,“order skipping” can be avoided.

FIG. 4 illustrates a method 200 according to another implementation ofthe invention. In method 200, the polishing endpoint for the productsubstrate 10 is determined based on the pre-determined removal rate fora monitor substrate and an initial thickness of the product substrate10.

Initially, the cassette 12 is delivered to the polisher 22 by thesubstrate processing system 20. The cassette 12 includes a monitorsubstrate 15 in addition to the set of regular device substrates 10. Themonitor substrate 15 can be a blank oxide-coated wafer. The monitorsubstrate is polished (step 210) while being in-situ monitored by thesystem 40 (step 220). The controller 32 determines the polishing rate ofthe monitor substrate (step 222) based on the signals received from themonitoring system 40. For each device substrate, the initial layerthickness is measured using the in-line metrology system 30 (step 224).The controller calculates the individual polishing time for eachindividual device substrate 10 (step 230), e.g., by dividing themeasured thickness by the polishing rate of the monitor substrate. Whenthe device substrate is being polished (step 240), the controller merelyhalts polishing (step 260) when the actual polishing time of the devicesubstrate becomes equal to the expected polishing time previouslydetermined by the controller 32 (step 250).

FIG. 5 illustrates a method 300 according to another implementation ofthe invention. In method 300, both the in-situ removal rate and thein-line measurement of the initial thickness are used to determine thepolishing time for the product substrate.

The initial thickness of a layer in a product substrate 10 is measuredby the in-line metrology device 30 (step 310). The controller 32determines an estimated polishing time for the product substrate 10(step 320) from the initial thickness of the substrate and from apreviously stored polishing rate, such as an empirically measuredpolishing rate or a polishing rate of a previously polished substrate asdetermine by the optical monitoring system 40. The product substrate 10is transferred to the polishing station 22 and polished while beingmonitored by the optical monitoring system 40 (step 330). After a presetamount of the estimated polishing time has occurred, e.g., about 80-90%of the estimated polishing time, the controller calculates the actualremoval rate of the polisher 22 from the intensity measurementsgenerated by the in-situ optical monitoring system 40. Then thecontroller 32 re-calculates the polishing endpoint time based on theactual removal rate and the initial thickness of the product substrate(step 340). When the polishing time reaches recalculated endpoint time,the controller halts polishing of the product substrate.

FIG. 6 illustrates a method 400 according to another implementation ofthe invention. In method 400, the initial thickness is used to calculatethe number of interference peaks expected during polishing. The endpointdetection system counts this number of interference peaks beforepolishing is halted.

The initial thickness of a device substrate 10 is pre-measured by thein-line metrology device 30 (step 410). The controller 32 calculates anamount to be removed from the upper surface of the substrate based onthe initial thickness and the target thickness of the layer on thesubstrate (step 420). The target thickness is a pre-determined value andis stored in the memory of the controller prior to the beginning of thepolishing process. An expected number of peaks in the interferencesignal is then calculated from the amount to be removed (step 425). Theproduct substrate is transferred to the polisher and polished whilebeing in-situ monitored by the optical monitoring system 40 (step 440).The series of intensity values from the reflected beam can be stored inthe controller, are analyzed, and the number of peaks in the sinusoidalsignal is counted (step 430). When the desired number of peaks has beencounted (step 440), polishing is halted. The number of peaks can be afraction, in which case the endpoint detection system can continuepolishing for a partial interference cycle, e.g., by monitoring theslope or other features of the interference signal, or by calculatingthe average duration of the previous interference cycles and polishingpast an interference peak for a time equal to the fraction of theaverage duration. Alternatively, the desired thickness to be removed canbe stored, and the controller can calculate the amount of thicknessremoved from the number of peaks in the intensity signal.

One problem in CMP endpoint detection is the influence of the refractiveindex variation and thickness of the underlying layers on the tracesmeasured by the in-situ monitoring system 40. Specifically, when thethickness or the refractive index of the underlying layers vary, thecorrelation between the signal from the monitoring system 40 and thethickness of the substrate becomes distorted. For example, a largevariation in the thickness of an underlying nitride layer may result ina large post-CMP variation of the oxide layer thickness. Although thethickness of the nitride layer can be consistent within a given lot,there can be large lot-to lot variations.

FIG. 7 illustrates a method 500 according to another implementation ofthe invention. In method 500, the thickness of an underlying layer isused to adjust the endpoint algorithm.

The in-line metrology device 30 is used to measure the thickness of theunderlying nitride layer of the first substrate from the cassette (step510). The thickness of the underlying layer is used by the controller tocalculate an overpolish time (step 520), and this overpolish time isused to adjust the endpoint control timing (step 530). The substrate 10is polished while being in-situ monitored by the monitoring system 40(step 540). The polishing endpoint is detected as normal (step 550), butpolishing is not halted immediately. Instead, the controller 32 causesthe polisher to continue to polish the substrate for the previouslycalculated overpolish time (step 555). After the overpolish time hasexpired, polishing is terminated (step 560).

In effect, the overpolish time corrects for the distortion in the tracecaused by variation in the underlying layer thickness that would causethe endpoint to be triggered too early. The relationship between theunderlying layer thickness and the necessary overpolish time can bedetermined empirically or theoretically, and can be stored as alook-up-table or the like.

Still another problem in CMP polishing is measuring the thickness of asubstrate that includes a thin layer, for example, an anti-reflectivecoating (ARC) layer covering the upper oxide layer. Typically, thethickness of the ARC is approximately about 150-300 Angstroms of TiN.Although the thickness of the ARC layer is usually fairly consistentwithin one cassette, the ARC layers can vary in thickness significantlyfrom one cassette to another. Specifically, in some lots, cassette-tocassette thickness variations of the ARC layer can exceed 100%. The TiNreflectivity and, consequently, the phase of the ISRM reflected beam 46vary significantly with the variations in the RCA layer thickness. Thiscan cause large differences in the amplitude of the signal from thein-situ monitoring system 40. On the other hand, a measurement of theARC layer with the in-line metrology device 35 alone may not be possiblebecause of the small thickness of the layer.

FIG. 8 illustrates a method 600 according to another implementation ofthe invention. In method 600, an optical endpoint detection criteria isselected that ensures that the ARC layer will be removed but thatresulting substrate will be somewhat underpolished (step 605). A firstdevice 17 substrate from the cassette 12 is polished using this opticalendpoint detection criteria (step 610). Since the ARC layer has beenremoved, the in-line metrology device 30 can be used, as describedabove, to measure the thickness of the layer remaining on theunderpolished substrate 17 at the metrology station 30 (step 615). Thisthickness is used to calculate an overpolish time based on the post-CMPthickness of the underpolished substrate 17 and a desired targetthickness (step 620). The target thickness is pre-determined data whichis stored in the controller prior to the beginning of the polishingprocess. The overpolish time can be calculated from a polish ratemeasured during by the in-situ monitoring system during polishing andthe desired thickness to be removed (calculated from the thicknessremaining and the target thickness). The timing portion of the endpointalgorithm is adjusted to use the overpolish time (step 630).

Subsequent device substrates 10 (i.e., after the first substrate 17) arepolished while being monitored by the optical monitoring system 40 (step640), the endpoint detection algorithm is used with the polishingcriteria previously calculated to provide an underpolished substrate(step 650), and after the optical endpoint criteria are detected, thepolisher continues to polish for the overpolish time (step 660).

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. A computer-implemented method for process control in chemicalmechanical polishing, comprising: polishing a monitor substrate from alot; monitoring the monitor substrate in-situ during the polishing stepto determine a lot representative removal rate; measuring an initialsubstrate thickness of a product substrate from the lot at a metrologystation; calculating an endpoint time from the initial substratethickness and the lot representative removal rate; polishing the productsubstrate; and stopping polishing of the product substrate at theendpoint time.
 2. The method of claim 1, further comprising: receiving aplurality of product substrates from the lot and the monitor substrateat a polisher in a cassette; measuring initial substrate thicknesses ofthe plurality of product substrates at the metrology station;calculating endpoint times from the initial substrate thicknesses andthe lot representative removal rate; polishing the plurality of productsubstrates; and stopping polishing of the plurality of productsubstrates at the endpoint times.
 3. The method of claim 1, whereinmeasuring an initial substrate thickness measures an initial thicknessof an oxide layer of the product substrate.
 4. The method of claim 1,wherein the monitor substrate comprises a blank oxide-coated substrate.